Remote DoS attack against SSH Secure Shell for Windows Servers Vulnerability


                                   U.S.S.R labs
Buenos Aires, Argentina

Remote DoS attack against SSH Secure Shell for Windows Servers Vulnerability

USSR Advisory Code:


Luciano Martins <>

SSH Secure Shell for Windows Server 2.4

SSH Secure Shell for Windows Server 2.5 available soon

Vendors Affected:
SSH Communications Security Corp

1. Discussion

UssrLabs has recently discovered a problem with Windows versions of sshd.
The problem lies with adjacent connection handling where the sshd is
unable to handle 64 simulataneous connections. As a result the sshd will
crash, and no services to the sshd will be accepted.

The problem lies in ssheloop.c where the assertion test fails.

The Event Log displays the following code after 64 connections are

FATAL ERROR: E:\src\lib\sshutil\ssheloop\win32\ssheloop.c:1597
(function name unavailable) Assertion failed:
ssh_adt_num_objects(ssh_eloop_events) < 64

No doubt, proper error handling techniques have not been implemented for
the SshEventLoop which ultimately causes the crash.

This results in a Denial of Service against the service in question.

2. Vendor Status:
Informed, Contacted.

3. Fix:
official fix will be available soon.

Related Links:

Underground Security Systems Research:

CrunchSp Product:


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